1. Field of the Invention
The present invention is generally related to switches that are intended for use in high temperature environments and, more particularly, to switches that have contact pads that are particularly selected to maintain their physical integrity and avoid being welded together when used to conduct electrical current at elevated temperatures.
2. Description of the Prior Art
Many different types of electrical switches are well known to those skilled in the art. Certain types of switches are provided with two stationary contacts and a movable contact that is able to move from a first position in electrical communication with a first stationary contact to a second position in electrical communication with a second stationary contact. Typically, one of the stationary contacts is a normally closed contact and the other stationary contact is a normally opened contact. A known construction of a switch comprises a first stationary contact on which a fine silver contact pad is attached or, alternatively, the stationary contact and the silver contact pad can be made entirely of fine silver or an alloy of silver. A second stationary contact is provided with a fine silver contact pad and supported by a nickel iron post. The movable contact pad is typically made of fine silver or silver cadmium oxide material. When the movable contact pad is disposed in contact, under force, against either of the stationary contact pads, elevated temperatures can create a problem situation.
The selection of contact pad material in switches known to those skilled in the art is generally a function of the type of application for which the switch is intended and the temperature of the environment in which the switch will be used. Contact pads in sealed precision switches are typically made of gold for low current applications below approximately 0.05 amperes. For applications in which the current is greater than 0.5 amperes, silver or silver alloy contact pads are generally used. Occasionally, a silver cadmium oxide alloy is used as part of the contact pad set in order to improve the resistance to contact welding at higher current loads. In addition, occasionally a silver alloy Fasaloy GAH, which is a silver-magnesium-nickel alloy, or similar material is used whenever structural strength is required in addition to good electrical performance.
During the development of a high temperature sealed switch, significant difficulty has been experienced with the performance of the normal materials typically used by those skilled in the art. Known combinations of electrical contact pad material failed to perform under the extreme conditions with elevated temperature. In this context, high temperature is taken to mean 750 degrees Fahrenheit continuous duty and 900 degrees Fahrenheit intermittent duty.
Attempts to use gold and some gold alloys have not been successful because gold softens significantly and becomes dimensionally unreliable at elevated temperatures. In addition, gold contact pads are subject to intermetallic bonding between contact surfaces when held in contact under a force provided by a spring at high temperatures. Another significant requirement of switches of this type is that the normally closed stationary contact requires structural strength as well as acceptable electrical performance. The normally closed contact carrier is subjected to the force of the spring in a snap switch when the movable contact is held in place against the normally closed stationary contact. For this reason, a fine silver stationary contact carrier has not been acceptable even in a switch rated for only 325 degrees Fahrenheit. A fine silver movable contact, in combination with a Fasaloy GAH alloy silver normally closed contact, does not work satisfactorily because at high temperatures the Fasaloy GAH alloy does not retain its strength and the structure of the normally closed stationary contact carrier can be deformed. If this deformation occurs, the position of the stationary contact pad does not accurately remain in its intended position and the operating characteristics of the switch are degraded significantly.
U.S. Pat. No. 4,706,383, which issued on Nov. 17, 1987 to Saffari, discloses an electrical contact assembly with composite contact construction. It teaches a method for producing a nonwelding contact assembly for an electrical switch using a composite contact material that is formed by extruding a metal oxide core surrounded by a metal or metal alloy sheath with good welding properties to form a wire of core material having a layer of the metal or metal alloy metallurgically bonded thereto. A segment of the wire is resistance welded to a contact carrier and coined to the desired contact shape after which the layer or the contact surface is sufficiently thin that it is oxidized to provide nonwelding characteristics after a minimum number of switch operations.
U.S. Pat. No. 5,334,811, which issued to Burgener et al on Aug. 2, 1994, discloses a switch with a laminated cover. The switch housing is made with a laminated cover that comprises an inner plate, an outer plate and a metallic sheet that is disposed therebetween. The metallic sheet is formed with a raised portion that defines a raised surface that is a deformable membrane. The protrusion formed in the metallic sheet is disposed through an opening in an outer place. A inner plate has an opening which is aligned with the opening of the outer plate and the three laminae are spot welded together to define a cover. The laminated cover is welded to a housing with a switching mechanism disposed therein.
Because of the problems caused by high temperature applications, it would therefore be significantly beneficial if a sealed switch could be provided with the capability of operating at high temperatures with the ability to withstand the elevated temperatures without having its electrical contact pads weld together or its contact carriers deformed.